The invention relates to an electrochemical gas sensor, an electrochemical gas detection cell, a system for detecting gas using such a cell and a filter which may be used in the cell. More particularly, the present invention is concerned with an electrochemical hydrogen sulfide sensor, an electrochemical hydrogen sulfide detection cell, a system using the cell and a filter which may be used in the cell. The invention is also concerned with a system for detecting gas which is constructed from modular parts.
Examples of prior art electrochemical sensors used in detection cells, which sensors include a sensing electrode, a reference electrode and a counter electrode are illustrated in respective U.S. Pat. Nos. 3,776,832 and 3,925,183 to Oswin et al. entitled respectively "Electrochemical Detection Cell" and "Gas Detecting and Quantitative Measuring Device" and issued respectively Dec. 4, 1983 and Dec. 9, 1975. The electrodes are of conventional types which are expensive and must be operated at conventional, relatively high voltage levels.
The U.S. Pat. No. 3,776,832 to Oswin et al., supra, describes a three-electrode electrochemical gas sensor which can be adapted to measure oxidizable or reducible gases such as H.sub.2 S, CO, Cl.sub.2 and hydrazine, as well as other gases. This particular known cell has two shortcomings. Firstly, it requires an aqueous electrolyte which has a limited shelf life due to evaporation of the electrolyte. Secondly, the temperature range within which the cell can operate is limited due to the possibility of freezing of the electrolyte.
The shortcomings noted above as a result of using an aqueous electrolyte have been recognized for some time. It has been proposed and disclosed in U.S. Pat. No. 4,169,779 to Tataria et al. entitled "Electrochemical Cell for the Detection of Hydrogen Sulfide" and issued on Oct. 2, 1979 to replace the aqueous electrolyte in a three-electrode cell with a nonaqueous electrolyte, which has a considerably lower freezing point and vapor pressure than aqueous electrolyte. The three-electrode sensor of this known electrochemical cell includes a counter electrode of gold or platinum black, relatively expensive materials. Moreover, the sensing electrode bias, with respect to that of the platinum air reference electrode is maintained in the range of from 300 mv. to 1100 mv., preferably in the range of from 500 mv. to 1000 mv. with the result that oxidizable interferents which may be present in many test environments cause erroneous responses, that is, result in decreased specificity.
Additional examples of electrochemical detection cells are disclosed and illustrated in the U.S. patents identified as follows:
______________________________________ U.S. Pat. No. 4,040,805 Nelms et al. August 9, 1977 U.S. Pat. No. 4,048,041 David et al. September 13, 1977 U.S. Pat. No. 4,127,461 Paulen November 28, 1978 U.S. Pat. No. 4,184,937 Tataria et al. January 22, 1980 U.S. Pat. No. 4,201,634 Setter May 6, 1980 U.S. Pat. No. 4,227,984 Dempsey et al. October 14, 1980 U.S. Pat. No. 4,235,097 Kring et al. November 25, 1980 U.S. Pat. No. 4,271,121 Diller et al. June 2, 1981. ______________________________________
Of particular interest as background prior art is the publication J. A. Plambeck, Electroanalytical Chemistry, Wiley-Interscience, pages 50-51, New York, N.Y. (1963) which is concerned with circuitry, generally referred to as a potentiostat, for maintaining a sensing electrode of an electrochemical cell at a fixed potential with respect to its reference electrode.